Leveling the characteristic of photographic emulsions



Dec. 19, 1944. c, BECKER I 2,365,376

LEVELING THE CHARACTERISTIC OF PHOTOGRAPHIC EMULSIONS Fi1ed'Aug 23, 1940 Patented Dec. 19, 1944 UNITED STATES PATENT orrlcs LEVELING THE cmnAc'mnlsrrc or rno'rocnarmc EMULSIONS Cariheinz Becker, Berlin-Wilmersdorf, Germany: vested in the Alien Property Custodian Application August 23, 1940, Serial No. 353,974 In Germany December 9, 1938 Claims. (01. 179'-100.3)

This invention relates generally to methods of recording sound upon film and in a more particular sense is concerned with a circuit to be used in making half-wave intensity records, wherein distortions duev to irregularities of the photographic film density characteristic curve are mintransparency is limited by the downward curvature of the photographic film density characteristic curve. For this reason, in order to obtain a sufiiciently large reduction of the interference or noise level, the use of the push-pull-class AB or the push-pull-class A recording circuit in which the minimum operating transparency lies above the curvature of the photographic characteristic. has been deemed necessary. Hitherto in making half-wave-intensity records, using the so-called sag as well as the so-called straight or direct method, minimum operating transparencies'were obtained which at the best amounted to about a fifth to an eighth of the maximum operating transparency. Due to the tonal gradation determined by the image, the photographic film density characteristic curve of the positive cannot be influenced, nor with the use of mere photographic means, can the slope of the negative film density curve be substantially increased beyond the hitherto obtainable degree. Moreover, to obtain reduction of the interference or noise level and volume. of sound of half-wave-transverse records,

it would be necessary to produce an infinite or even a negative slope of the negative film characteristiccurve. Y

By practice of the present invention, a practically linear transfer characteristic curve. with a residual background (no-signal) transparency of any desired density and with any desired reduction of interference or noise level is obtained while making halI-wave-intensity records. These crating voltages and the corresponding half-wave signal voltages. These additional voltages decrease in proportion to increase of the amplitude of the half-wave signal voltages and are so chosen that the product of the 'slope of the characteristic of the electric transfer device (the Kerr-cell and operating circuit) and the photographic blackening characteristic is a constant value for each amplitude.

In accordance with the present invention, to embody the principles above mentioned, an ordinary Kerr-cell is divided to provide two pairs of electrode plates, each pair being supplied with a threshold operating voltage in addition to the corresponding half-wave signal voltage and, during operation on the non-linear portion of thecurve of the photographic film density characteristic, with an additional incremental voltage varying whereby its magnitude is progressively larger for progressively smaller values of slope of the photographic film density characteristic curve.

An additional or incremental voltage 'satisfyof electron tubes having exponential characterisremarkable eflects are achieved by using a novel divided Kerr-cell having two pairs of plates and applying additional incremental voltages to each pair of plates depending on the amplitude of the recording signal, in addition to the threshold optics and by adding to the two half-wave signal voltages the voltages occuring at the anode resistances of said tubes.

Preferably the control circuits of the two sets of Kerr-cell electrodes are arranged whereby the source of the half-wave signal voltages serves also to control operation of the, exponential tubes,'the .anode voltages of which serve as threshold or operating voltages for the Kerr-cell electrodes 4 and the anode resistances of which are connected in series with the cell electrodes in each of the ,4

circuits. v

The accompanying drawing is a circuit diagram illustrating an application of the principlesor to resistances l4 and I5 which in turn are connected to the potential source 16 above mentioned. The currents flowing through the two circuits are such that? threshold operating volt-' I 8 and 9, and the resistances IE and I provided in the operating circuits of the two pairs of Kerrcell plates, serve as the anode resistances of the tubes 8 and 9. The potentials appearing in the anode resistances id and it are added to the ground in plane parallelism, with a metallic sur facing layer by cathode atomising, galvanisation or the like. After surfacing in this manner, a

. narrow linear area near the middle'of each of threshold operating voltages derived from the voltages necessary for compensation of irregularities in the photographic film density curve as above discussed.

The half-wave voltages are applied to the ire-l sistances 6 and l in a manner such that the points I ands are negative relative to the point 2 which accordingly is positive whereby each half-wave produces a negative voltage impulse at the control grid of the corresponding ex ponential tube. Due to the logarithmic 'charac-- teristic curve of tubes of this type, the anode currents passingthrough the tubes are not proportional to the amplitudes of the halt-wave signals applied to the control grids, but instead an increase of the half-wave signal amplitudes, produces a progressively smaller increase of the anode current until ultimately the anode current no longer increases. .From this fact, it will be evident that the additional voltage cannot exceed an upper limiting value, or in other words, with an increasing amplitude of the half-wave signal voltage, the additional or incremental voltage decreases proportionally as this amplitude increases.

A Kerr-cell scribed circuit, the manufacture of which is very diilicult. A reason for this d fliculty lies in the fact that when making records the plates of the usual type of Kerr-cells tend to vibrate under the influence of the electric alternating current fields, thereby preventing the .production of a record free from distortion. Furthermore, it is necessary that the two electrodes be maintained rigidly in accurately spaced parallelism. Under usual conditions of manufacture these requirements cannot be fulfilled when using the type of electrodes hitherto known, especially those consisting of metal plates and-metal blocks.

It has now been found that the Kerr-cell f r use'in the above described circuit can be constructed in a manner such that the two pairs of electrodes are'iormed upon the surfaces of two glass and porcelain plates, ground in plane parallelism, which can be separated from each other byinsulating spacers of similar nature. In this instance, the surfaces facing each other are provided with a thin metal layer, each divided in the middleof the plate, thus providing pairs of coelongate rectangular glass or porcelain plates,

th plates is removed, parallel .to the shorter edge of the plate, whereby two metal surfaced areas, electrically separated from each other, are provided side by side upon each plate. The two plates thus prepared are mounted in the Kerrcell with the metal layers facing each other and maintained in proper spaced relationship by plane parallel insulating spacers.

Double Kerr-cells of this type, in addition to ruggedness of construction have the advantage that the plates comprising each of the two pairs of electrodes are positively held in equally spaced relationship, thus from the standpoint of electrical characteristics the two pairs of plates are equivalent.

What I claim is:

-l. The method of compensating for the nonlinear density characteristic curve of photographic film when recording half-wave intensity records by apparatus. including a Kerr cell having a pair, of plates mounted rigidly in accurately placed parallelism, which method comprises impressing a portion of the operating voltage applied to the plates of the cell as a negative bias on the grid of an electron tube having an. exponential characteristic, and adding the anode voltage of said tube to the 'operating voltage oi the cell.

2.- Apparatus for half-wave photographic sound recording comprising a Kerr cell having a pair oi plates mounted rigidly in accurately spaced parallelism, means for supplying to the pair of plates a half-wave voltage modulated in response to the sound to be recorded, an electron tube' having exponential characteristics, the grid cir' cult of said tube being coupled to be negatively biased by a portion of the half-wave voltage, and

having four electrically independ- 'en t plates is required in utilizing the above dethe anode circuit of said tube being connected to the pair of plates in seriesadding with said halfwave voltage whereby the anode voltage compensates for the non-linear portion of the photographic film density characteristic curve.

. 3. In the process of recording sound photographically using a divided, Kerr cell light valve having two aligned pairs of rigidly mounted accurately spaced parallel plates having modulated half-wave voltages applied to the respective pairs of plates, the improvement which comprises separately applying a portion of each of the halfwave voltages as a negative bias to the control grid of each of a pair of electron tubes having an exponential characteristic, and adding to the .correspondinghalf-wave voltages the anode voltages of said tubes.

4. Apparatus for half-wave photographic sound recording comprising a divided Kerr cell having two alignedv pairs of rigidly mounted accurately spaced parallel plates, means for supplying to the pairs of plates, respectively, half wave voltconnected to the pairs of plates in series adding with saidhalf-wave voltages whereby the anode voltages compensate for the non-linear portion of the photographic film density characteristic curve.

in response to the sound to be recorded, a pair of electron tubes having exponential characteristics,

the grid circuits of said tubes being coupled to be negatively biased by a portion of the'respective half-wave voltages, and the anode circuits of the tubes being respectively connected to the pairs of coatings in series adding with said half-wave voltages whereby the anode voltages compensate ior the non-linear portion of the photographic film density characteristiccurve.

C ARLHEINZ BECKER. 

